[Dotenv] support setting default env var values

Author: xabbuh

Dotenv.php

@@ -432,7 +432,7 @@ private function resolveVariables(string $value): string
             (?!\()                             # no opening parenthesis
             (?P<opening_brace>\{)?             # optional brace
             (?P<name>'.self::VARNAME_REGEX.')? # var name
-            (?P<default_value>:-[^\}]++)?      # optional default value
+            (?P<default_value>:[-=][^\}]++)?   # optional default value
             (?P<closing_brace>\})?             # optional closing brace
         /x';
 
@@ -469,6 +469,10 @@ private function resolveVariables(string $value): string
                 }
 
                 $value = substr($matches['default_value'], 2);
+
+                if ('=' === $matches['default_value'][1]) {
+                    $this->values[$name] = $value;
+                }
             }
 
             if (!$matches['opening_brace'] && isset($matches['closing_brace'])) {

Tests/DotenvTest.php

@@ -166,6 +166,10 @@ public function getEnvData()
             ["FOO=BAR\nBAR=\${NOTDEFINED:-TEST}", ['FOO' => 'BAR', 'BAR' => 'TEST']],
             ["FOO=\nBAR=\${FOO:-TEST}", ['FOO' => '', 'BAR' => 'TEST']],
             ["FOO=\nBAR=\$FOO:-TEST}", ['FOO' => '', 'BAR' => 'TEST}']],
+            ["FOO=BAR\nBAR=\${FOO:=TEST}", ['FOO' => 'BAR', 'BAR' => 'BAR']],
+            ["FOO=BAR\nBAR=\${NOTDEFINED:=TEST}", ['FOO' => 'BAR', 'NOTDEFINED' => 'TEST', 'BAR' => 'TEST']],
+            ["FOO=\nBAR=\${FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST']],
+            ["FOO=\nBAR=\$FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST}']],
         ];
 
         if ('\\' !== \DIRECTORY_SEPARATOR) {